White film for surface light source reflecting members

ABSTRACT

A white film for a reflecting structure for surface light sources, which contains voids inside it and has a light stabilizer-containing coating film formed on at least one surface of it, is aged little with time and its brightness lowers little even when used for a long time. The white film ensures good image quality of display screens for a long time, and it is favorable to reflecting sheets and reflectors in edge light-type and direct back light-type surface light sources for liquid crystal display screens.

TECHNICAL FIELD

[0001] The present invention relates to improvements on white film for areflecting structure for surface light source. More precisely, theinvention relates to a structure of white film which is used inreflecting sheets and reflectors for edge light-type and direct backlight-type, surface light sources for liquid crystal display screens,and of which the brightness lowers little when used for a long time.

BACKGROUND ART

[0002] For lighting units for liquid crystal display screens, widelyused is a edge light system in which a cold cathode-ray tube serving asthe source of light is disposed at the edge of a light guide plate (JP-A62104/1988). In the lighting system of the type, a reflector is disposedaround the cold cathode-ray tube for increasing the lighting efficiencytherein, and a reflecting sheet is disposed below the light guide platefor efficiently reflecting the light having been diffused through thelight guide plate toward a liquid crystal display screen. In the system,these have the function of reducing the loss of light from the coldcathode-ray tube and increasing the brightness of the liquid crystaldisplay screen. However, the edge light system could not increase thebrightness of recent wide screens of liquid crystal TVs, for which,therefore, a direct back light system is being employed. In the directback light system, cold cathode-ray tubes are aligned in parallel belowa liquid crystal display screen, and they are above a reflecting sheetin parallel with each other. The reflecting sheet may be flat or may besemi-circularly shaped to partly cover each cold cathode-ray tube.

[0003] For use in the surface light source for such liquid crystaldisplay screens, the reflector and the reflecting sheet (these aregenerically referred to as a reflecting structure for surface lightsource) are required to have high reflectivity, for which generally usedare films with white dye or pigment added thereto or films with finevoids therein either alone or as laminated with any of metal or plasticplates. The films with fine voids therein are widely used as theirability to improve screen brightness and uniformity is good. They aredisclosed in, for example, JP-A 322153/1994 and 118433/1995.

[0004] The recent expansion of the use of liquid crystal display screensis remarkable. For example, they are being widely used not only inconventional notebook-size personal computers but also in desk-toppersonal computers, liquid-crystal TVs, mobile telephone displays,various game computers, etc. With the expansion of their use, thescreens are desired to have increased brightness and increased fineness,for which the power of the lighting source is increased and the numberof the light source lamps is increased. In addition, for satisfying therequirement of long-term lighting of wide screens of liquid crystal TVs,they are required to have more increased brightness and durability. Inparticular, when the screens are lighted by a direct back light system,they shall directly receive the light from the light source. For these,the durability of the reflecting sheet is required to be higher.However, reflectors and reflecting sheets of conventional film areproblematic in that the film is aged and yellowed when used for a longtime and therefore its reflectivity is lowered with time and the screenbrightness is after all lowered.

[0005] The present invention is to solve the problems noted above, andits object is to provide a reflecting structure for surface light sourceof which the brightness lowers little with time even when used for along time and which ensures high-quality images for a long time.

DISCLOSURE OF THE INVENTION

[0006] To solve the problems as above, the subject matter of theinvention resides in a white film for surface light source reflectingstructures, which contains voids inside it and has a lightstabilizer-containing coating film formed on at least one surface of it.

BRIEF DESCRIPTION OF THE DRAWING

[0007]FIG. 1 is a vertical cross.-sectional view showing the outlinestructure of a device for measuring the brightness of the surface lightsource in the invention.

BEST MODES OF CARRYING OUT THE INVENTION

[0008] The white film of the invention is not specifically defined sofar as it is apparently white, including, for example, thermoplasticfilms with any of organic or inorganic dye or fine particles addedthereto; films formed by mixing a film-forming resin component and aresin not miscible with it, and/or organic or inorganic particles,melt-kneading the resulting mixture, and stretching it at least in onedirection to thereby make the film have fine voids therein; foam filmsformed through melt extrusion with foaming particles added thereto; andfoam films formed through foaming extrusion with a vapor such as carbondioxide introduced thereinto. Especially for the use of the invention,preferred are films formed by mixing a film-forming resin component anda resin not miscible with it, and/or organic -or inorganic particles,melt-kneading the resulting mixture, and stretching it at least in onedirection to thereby make the film have fine voids therein, as theirreflectivity is higher and their brightness is higher. More preferredare composite films formed through coextrusion of laminating an organicor inorganic fine particles-containing thermoplastic resin film on atleast one surface of a film that contains fine voids therein, followedby stretching the laminate film to thereby make it have finer voids inthe surface layer than in the inside thereof.

[0009] The thermoplastic resin to form the film is not specificallydefined so far as it forms films through melt extrusion. Its preferredexamples are polyesters, polyolefins, polyamides, polyurethanes, andpolyphenylene sulfides. In the invention, especially preferred arepolyesters as they have good dimensional stability and good mechanicalproperties and they do not almost absorb visible light.

[0010] Examples of the polyesters are polyethylene-terephthalate(hereinafter referred to as PET), polyethylene 2,6-naphthalenedicarboxylate (hereinafter referred to as PEN),polypropylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexylenedimethylene terephthalate. Needless-to-say, thesepolyesters may be homopolymers or copolymers, but are preferablyhomopolymers. For the copolymers, the comonomer includes, for example,aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclicdicarboxylic acids, and diols having from 2 to 15 carbon atoms. Theirexamples are isophthalic acid, adipic acid, sebacic acid, phthalic acid,sulfonate base-containing isophthalic acid, and their ester-formingcompounds, diethylene glycol, triethylene glycol, neopentyl glycol, andpolyalkylene glycols having a molecular weight of from 400 to 20,000.

[0011] These polyesters may contain various additives such asheat-resistant stabilizer, antioxidant stabilizer, organic lubricant,organic and inorganic fine particles, light-proofing agent, antistaticagent, nucleating agent and coupling agent, not interfering with theeffect of the invention.

[0012] One preferred embodiment of the invention is described below, inwhich polyester is used as the white film base material. To whiten thepolyester film, for example, employable are a method of adding theretovarious types of white dye or pigment; and a method of forming finevoids inside it as in the above. For attaining better results in theinvention, preferred is the method of forming fine voids inside thefilm. For forming such fine voids inside it, for example, employable are(1) a method of adding a foaming agent to the resin to thereby make theresin film foamed by heat in the step of extrusion or film processing orfoamed through chemical decomposition; (2) a method of adding a vaporsuch as carbon dioxide or a vaporizable substance to the resin during orafter its extrusion to thereby make the resin film foamed; (3) a methodof adding a thermoplastic resin not miscible with polyester to theresin, melt-extruding the resin mixture, and monoaxially or biaxiallystretching the resin film; and (4) a method of adding organic orinorganic fine particles to the resin, melt-extruding the mixture, andmonoaxially or biaxially stretching the resin film. In the invention,the fine voids formed in the film are to increase the reflectiveinterface therein, for which, therefore, preferred is the method (3) or(4).

[0013] The size of the voids formed in the methods as above (this is thesize of the cross section of the voids cut in the direction -of thethickness of the film) preferably falls between 0.5 μM² and 50, morepreferably between 1 μm² and 30 μM², in view of the increased brightnessof the film. The cross-sectional profile of the voids may be circular oroval. Preferably, the film is so constituted that at least one voidexists everywhere in its vertical direction running from the top face tothe back face thereof. When the film is formed into a reflecting sheetthe light from a light source enters it through the film surface, and itis the best that all the incident light having reached the reflectingsheet is entirely reflected by the voids inside the film. In fact, somelight will pass through the film, and it shall be a light loss. Tocompensate it, the surface of the film opposite to the surface thereofthat receives light (facing the light source) is preferably coated withmetal such as aluminium or silver through vapor deposition. In addition,for reducing the light loss through the fine voids-containing polyesterfilm, it is also desirable that the surface of the film is coated with alayer that contains fine voids formed by organic or inorganic fineparticles. The surface layer may be formed by co-extruding a polyesterresin that contains organic or inorganic fine particles, along with theresin for the fine voids-containing film, followed by stretching theresulting composite film at least in one direction. Preferably, thevoids in the surface layer are smaller than those in the inner layer ofthe composite film for increasing the brightness of the film. The ratio(void size in surface layer/void size in inner layer) is notspecifically defined, but preferably falls between 0.05 and 0.8, morepreferably between 0.07 and 0.7, most preferably between 0.1 and 0.6.The void size can be controlled by controlling the size of the particlesto be added to film-forming resins.

[0014] Now described hereinunder are the resin not miscible withpolyester resin, and the organic or inorganic particles to be added tothe surface layer and the inner layer, which are to form voids inpolyester films. The resin not miscible with polyester film (hereinafterreferred to as immiscible resin) is a thermoplastic resin exceptpolyester, and this can disperse in polyester, forming particlestherein. Preferred examples of the resin of the type are polyolefinresins such as polyethylene, polypropylene, polybutene,polymethylpentene; as well as polystyrene resins, polyacrylate resins,polycarbonate resins, polyacrylonitrile resins, polyphenylene sulfideresins, and fluororesins. These may be homopolymers or copolymers, andtwo or more different types of these may be combined for use herein.Especially preferred are resins that yield a great critical surfacetension difference from polyester and hardly deform in heat treatmentafter stretching. For these, preferred are polyolefin resins, and morepreferred is polymethylpentene. The content of the immiscible resin tobe in the white film is not specifically defined, and may be suitablydetermined so that the film is not broken while formed and thebrightness of the film can be increased by the voids formed from thenuclei of the immiscible resin in the film. In general, it fallspreferably between 3 and 35% by weight, more preferably between 4 and30% by weight, most preferably between 5 and 25% by weight. If thecontent is smaller than 3% by weight, the brightness of the film couldnot increase so much; but if larger than 35% by weight, the film may bebroken while formed.

[0015] The inorganic fine particles to be added to the inner layerand/or the surface layer are preferably those that may be nuclei bythemselves to form voids in the layers. For these, for example, usableare calcium carbonate, magnesium carbonate, zinc carbonate, titaniumoxide (anatase type, rutile type), zinc oxide, barium sulfate, zincsulfide, basic zinc carbonate, titanium mica, antimony oxide, magnesiumoxide, calcium phosphate, silica, alumina, mica, talc, kaolin. Of those,especially preferred are calcium carbonate and barium carbonate thatabsorb little visible light falling within a range of from 400 to 700nm. Particles that absorb visible light are problematic as thebrightness of the film containing them is lowered. The organic fineparticles are preferably those not fusing in melt extrusion. For these,especially preferred are crosslinked particles of crosslinked styrene orcrosslinked acryl. The organic fine particles may be hollow. One or moredifferent types of these fine particles may be used herein either singlyor as combined. The particle size is not specifically defined, generallyfalling between 0.05 and 15 μm, preferably between 0.1 and 10 μm, morepreferably between 0.3 and 5 μm. If their size is smaller than 0.05 μm,the particles could not yield good voids in films; but, on the contrary,if larger than 15 μm, it is unfavorable since the film surface may betoo much roughened. In case where fine particles are in the surfacelayer and immiscible resin is in the inner layer, it is desirable thatthe size of the voids formed from the nuclei of the fine particles inthe surface layer is smaller than that of the size of the voids formedin the inner layer in order that the brightness of the film is higher.Also preferably, the content of the fine particles in the white filmfalls between 1 and 30% by weight, more preferably between 2 and 25% byweight, most preferably between 3 and 20% by weight. If the content issmaller than 1% by weight, the brightness of the film could not beincreased to a desired degree; but, on the contrary, if larger than 30%by weight, the film may be broken while formed.

[0016] The specific gravity of the voids-containing white film, which isa criterion of the void content of the film, is preferably not smallerthan 0.1 but smaller than 1.3. If the specific gravity of the film issmaller than 0.1, it is problematic in that the mechanical strength ofthe film is low and the film is readily folded and is difficult tohandle. On the other hand, if the specific gravity of the film is largerthan 1.3, the void content thereof is too low, and therefore thereflectivity of the film is low and the brightness thereof may beinsufficient. In case where the film-forming thermoplastic resin ispolyester, the lowermost limit of the specific gravity of the film ispreferably 0.4. If its specific gravity is smaller than 0.4, the voidcontent of the film is too high, and it is problematic in that the filmis frequently broken while formed.

[0017] The reflecting structure for surface light source is a tabularstructure that is combined with a surface light source for lightreflection thereon, as so mentioned hereinabove. Concretely, it includesreflecting sheets for edge lights-type surface light sources for liquidcrystal display screens, reflecting sheets for direct back light-typesurface light sources, and reflectors around cold cathode-ray tubes. Forthe reflecting structure for surface light source of the type; thedegree of whiteness of the reflecting sheet is preferably higher forbettering the color tone of screens, and bluish reflecting sheets arepreferred to yellowing ones. Taking this into consideration, it isdesirable to add a fluorescent brightener to the white film. Thefluorescent brightener may be any commercially-available one, including,for example, Uvitex (by Ciba-Geigy), OB-1 (by Eastman), TBO (bySumitomo'Seika), Keikol (by Nippon Soda), Kayalite (by Nippon Kayaku),and Leucopoor EGM (by Clariant Japan). Preferably, the content of thefluorescent brightener in the white film falls between 0.005 and 1% byweight, more preferably between 0.007 and 0.7% by weight, mostpreferably between 0.01 and 0.5% by weight. If its content is smallerthan 0.005% by weight, the fluorescent brightener may be ineffective;but if larger than 1% by weight, it is unfavorable since too muchfluorescent brightener rather yellows the white film. In case where thewhite film is a composite film, the fluorescent brightener is morepreferably added to the surface layer of the film.

[0018] In the invention, at least one surface of the white film must becoated with a light stabilizer-containing coating layer. The lightstabilizer may be any of organic light stabilizers of, for example,hindered amines, salicylates, benzophenones, benzotriazoles,cyanoacrylates, triazines, benzoates, oxalic anilides; and inorganiclight stabilizers of, for example, sol-gel compounds. Preferred examplesof the light stabilizer for use in the invention are mentioned below.Needless-to-say, these are not limitative.

[0019] Hindered amines: bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate,polycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylp iperidine;

[0020] Salicylates: p-t-butylphenyl salicylate, p-octylphenylsalicylate;

[0021] Benzophenones: 2,4-dihydroxybenzophenone,2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone,2,2′,4,4′-tetrahydroxybenzophenone,2,2′-dihydroxy-4-methoxybenzophenone,2,2′-dihydroxy-4,4′-dimethoxybenzophenone,bis(2-methoxy-4-hydroxy-5-benzoylphenyl)methane;

[0022] Benzotriazoles: 2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotria zole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-5′-t-octylphenol)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole,2,2′-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol],2-(2′-hydroxy-5′-methacryloxyphenyl)-2H-benzotriazole,2-[2′-hydroxy-3′-(3″,4″,5″,6″-tetrahydrophthalimidomethyl)-5′-methylphenyl]benzotriazole,2-(2′-hydroxy-5-acryloyloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-acryloylethylphenyl)-5-chloro-2H-benzotriazole;

[0023] Cyanoacrylates: ethyl-2-cyano-3,3′-diphenylacrylate;

[0024] Others than the above: nickelbis(octylphenyl) sulfide,[2,2′-thiobis(4-t-octylphenolato)]-n-butylaminenickel, nickelcomplex-3,5-di-t-butyl-4-hydroxybenzyl phosphate monoethylate, nickeldibutyldithiocarbamate, 2,4-di-t-butylphenyl3′,5′-di-t-butyl-4′-hydroxybenzoate, 2,4-di-t-butylphenyl3′,5′-di-t-butyl-4′-hydroxybenzoate, 2-ethoxy-2′-ethyloxalic acidbisanilide, 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)oxy]-phenol.

[0025] Of the examples mentioned above, at least any of hindered amines,benzophenones and benzotriazoles are preferred for use in the invention,more preferably, these are combined for use herein.

[0026] In the invention, the light stabilizer to be in the coating layeris preferably mixed with any other resin component for facilitating theformation of the coating layer. Specifically, one preferred embodimentfor it comprises dissolving or dispersing the resin component and thelight stabilizer in an organic solvent capable of dissolving the resincomponent and the light stabilizer, or in water, a mixture of two ormore different types of organic solvents, or a mixture of organicsolvent/water to prepare a solution or dispersion that serves as thecoating liquid for the layer. Needless-to-say, the resin component andthe light stabilizer may be separately dissolved or dispersed in such anorganic solvent, water, an organic solvent mixture or a mixture oforganic solvent/water, and the resulting solutions or dispersions may bemixed in any desired ratio to be the coating liquid. Also preferred ispreparing a copolymer of the light stabilizer component and the resincomponent followed by directly using the copolymer for the coatingmaterial. Needless-to-say, the copolymer may be dissolved in an organicsolvent, water, a mixture of two or more different types of organicsolvent, or a mixture of organic solvent/water to prepare a solution forthe coating liquid. The resin component to be mixed or copolymerizedwith the light stabilizer is not specifically defined. Its examples arepolyester resins, polyurethane resins, acrylic resins, methacrylicresins, polyamide resins, polyethylene resins, polypropylene resins,polyvinyl chloride resins, polyvinylidene chloride resins, polystyreneresins, polyvinyl acetate resins, and fluororesins. These resins may beused either singly or as combined to be a copolymer or mixture of two ormore of them.

[0027] Of the resin components mentioned above, preferred for thecoating layer are acrylic resins or methacrylic resins, and morepreferred are acrylic resins or methacrylic resins copolymerized withthe light stabilizer component. For, copolymerizing these, it isdesirable that the acrylic or methacrylic monomer component iscopolymerized with the light stabilizer monomer component.

[0028] For the light stabilizer monomer component, for example,preferred are reactive benzotriazole monomers, reactive hindered aminemonomers, and reactive benzophenone monomers. Not specifically defined,the benzotriazole monomers may be any ones having a benzotriazoleskeleton and having an unsaturated bond. Their examples are2-(2′-hydroxy-5-acryloyloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole, and2-(2′-hydroxy-3′-t-butyl-5′-acryloylethylphenyl)-5-chloro-2H-benzotriazole.Similarly, the reactive hindered amine monomers and the reactivebenzophenone monomers may be any ones having a hindered amine orbenzophenone skeleton and having an unsaturated bond. Examples of thereactive hindered amine monomers arebis(2,2,6,6-tetramethyl-4-piperidyl-5-acryloyloxyethylpheny 1) sebacate,polycondensate of dimethylsuccinate/l-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-acryloyloxyethylphenylpiperidine,bis(2,2,6,6-tetramethyl-4-piperidyl-5-methacryloxyethylphen yl)sebacate, polycondensate of dimethylsuccinate/l-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-methacryloxyethylphenylpiperidine,bis(2,2,6,6-tetramethyl-4-piperidyl-5-acryloylethylphenyl) sebacate, andpolycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-acryloylethylphenylpiperidine.Examples of the reactive benzophenone monomers are2-hydroxy-4-methoxy-5-acryloyloxyethylphenylbenzophenone,2,2′,4,4′-tetrahydroxy-5-acryloyloxyethylphenylbenzophenone,2,2′-dihydroxy-4-methoxy-5-acryloyloxyethylphenylbenzopheno ne,2,2′-dihydroxy-4,4′-dimethoxy-5-acryloyloxyethylphenylbenzo phenone,2-hydroxy-4-methoxy-5-methacryloxyethylphenylbenzophenone,2,2′,4,4′-tetrahydroxy-5-methacryloxyethylphenylbenzophenon e,2,2′-dihydroxy-4-methoxy-5-acryloylethylphenylbenzophenone, and2,2′-dihydroxy-4,4′-dimethoxy-5-acryloylethylphenylbenzophe none.

[0029] Examples of the acrylic or methacrylic monomer component or itsoligomer component to be copolymerized with the light stabilizer monomercomponent are alkyl acrylates, alkyl methacrylates (the alkyl groupincludes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, t-butyl, 2-ethylhexyl, lauryl, stearyl and cyclohexyl groups),and crosslinking functional group-having monomers such as those havingany of a carboxyl group, a methylol group, an acid anhydride group, asulfonic acid group, an amido group, a methylolated amido group, anamino group, an alkylolated amino group, a hydroxyl group, and an epoxygroup. Apart from these, the light stabilizer monomer component may becopolymerized into copolymers with any of acrylonitrile,methacrylonitrile, styrene, butyl vinyl ether, maleic acid, itaconicacid and dialkyl esters thereof, methyl vinyl ketone, vinyl chloride,vinylidene chloride, vinyl acetate, vinylpyridine, vinylpyrrolidone,vinyl group-having alkoxysilanes, and unsaturated polyesters.

[0030] The copolymerization ratio of the light stabilizer monomercomponent to the comonomer is not specifically defined, and the lightstabilizer monomer component may be copolymerized with one or more suchcomonomers in any desired ratio. Preferably, however, the ratio of thelight stabilizer monomer component to the comonomer is at least 10% byweight, more preferably at least 20% by weight, most preferably at least35% by weight. Needless-to-say, the light stabilizermonomer componentmayform a homopolymer. The molecular weight of the polymer is notspecifically defined, but is generally at least 5,000, preferably atleast 10,000, most preferably at least 20,000 in order that the coatinglayer formed of the polymer could be tough. The polymer is formed intothe coating layer, after dissolved or dispersed in an organic solvent,water or a mixture of organic solvent/water. Apart from these, othercommercially-available hybrid-type light stabilizer polymers, forexample, U-Double (by Nippon Shokubai) may also be used herein.

[0031] The thickness of the light stabilizer-containing coating layer isnot specifically defined, preferably falling between 0. 5 and 15 μm,more preferably between 1 and 10 μm, most preferably between 2 and 7 μm.If the thickness is smaller than 0.5 μm, the durability of the coatinglayer maybe low; but, on the contrary, if larger than 15 μm, thebrightness of the layer-coated white film may lower.

[0032] The light stabilizer-containing coating layer may be directlyformed on the white film. However, when the adhesiveness between thelayer and the film is low, the film is preferably corona-discharged orundercoated. For undercoating it, employable is any of an in-linecoating process in which the white film is undercoated in the line whereit is formed, or an off-line coating process in which the substrate isafter separately prepared in a film-forming line, undercoated in adifferent coating line. The undercoating material is not specificallydefined, and may be selected from any desired ones. Its preferredexamples are copolyester resins, polyurethane resins, acrylic resins,methacrylic resins, and various coupling agents.

[0033] The light stabilizer-containing coating layer may be formed onthe white film in any desired manner. For it, for example, employable isany method of gravure coating, roll coating, spin coating, reversecoating, bar coating, screen coating, blade coating, air knife coatingor dipping. In case where the coating layer is cured, after formed,employable is any known curing method. For example, the coating layermay be cured through exposure to heat or to active rays such as UV rays,electron rays or radiations, or exposure to any of these combinations.In curing it, a curing agent such as a crosslinking agent is preferablyused. For forming the coating layer, employable is any of an in-linecoating process in which the white film is coated with the coating layerin the line where it is formed, or an off-line coating process in whichthe substrate is, after separately prepared and processed for crystalorientation therein in a film-forming line, coated with the coatinglayer in a different coating line.

[0034] Preferably, the white film of the invention has a meanreflectance of at least 85%, more preferably at least 87%, even morepreferably at least 90%, when measured on the lightstabilizer-containing coating layer thereof exposed to light having awavelength of from 400 to 700 nm. If the mean reflectance of the whitefilm is smaller than 85%, the screen brightness of some liquid crystaldisplays comprising the white film may be low, depending on the type ofthe displays.

[0035] Also preferably, the white film of the invention has a degree ofglossiness of at most 60%, more preferably at most 50%, most preferablyat most 40%, when measured on the light stabilizer-containing coatinglayer thereof. If the glossiness of the white film is larger than 60%,the screen brightness of some liquid crystal displays comprising thewhite film may be low, depending on the viewing angle to the screen.

[0036] In the invention, the light stabilizer-containing coating layermay contain various additives not interfering with the effect of theinvention. The additives are, for example, organic and/or inorganic fineparticles, fluorescent brightener, crosslinking agent, heat-resistantstabilizer, antioxidant stabilizer, organic lubricant, antistatic agent,nucleating agent, and coupling agent.

[0037] Adding organic and/or inorganic fine particles to the coatinglayer is especially preferred, as making it easy to control theglossiness of the layer-coated surface of the white film within thedefined range as above. For the inorganic fine particles, for example,usable are silica, alumina, titanium oxide (anatase type, rutile type),zinc oxide, barium sulfate, calcium carbonate, zeolite, kaolin, andtalc. For the organic particles, for example, usable are crosslinkedstyrene and crosslinked acryl. Preferably, the particle size of theorganic and/or inorganic fine particles falls between 0.05 and 15 μM,more preferably between 0.1 and 10 μm. If smaller than 0.05 μm, theeffect of the particles to reduce the surface glossiness may be low;but, on the contrary, if larger than 15 μm, it is unfavorable since thesurface of the white film may be too much roughened and the particlesmay readily drop off. The content of the particles in the coating layerpreferably falls between 0.5 and 50% by weight, more preferably between1 and 40% by weight, most preferably between 2 and 30% by weight. If thecontent is smaller than 0.5% by weight, the effect of the particles toreduce the surface glossiness maybe low; but, on the contrary, if largerthan 50% by weight, it is unfavorable since the coating layer isdifficult to form and, in addition, the surface of the white film may betoo much roughened and the particles may readily drop off.

[0038] Adding a fluorescent brightener to the coating layer is morepreferred, as improving the whiteness and the appearance of thelayer-coated white film. The fluorescent brightener may be the same asthose mentioned hereinabove that can be added to the white film. Thefluorescent brightener content of the coating layer preferably fallsbetween 0.01 and 2% by weight, more preferably between 0.03 and 1.5% byweight, most preferably between 0.05 and 1% by weight. If its content issmaller than 0.01% by weight, the fluorescent brightener may beineffective; but if larger than 2% by weight, it is unfavorable sincethe coating layer may be rather yellowed and the durability thereof maylower.

[0039] Preferably, the thickness of the white film in the inventionfalls between 10 and 500 μm, more preferably between 20 and 300 μm. Ifthe thickness thereof is smaller than 10 μm, the reflectivity, thewhiteness and the appearance of the white film may not reach thepractical level, and the white film may be difficult to handle. On theother hand, if thicker than 500 μm, the white film may be too heavy andmaybe therefore unsuitable for a reflecting structure for surface lightsources to be in liquid crystal displays, and, in addition, its cost mayincrease. In case where the white film is a composite film, the ratio ofsurface layer/inner layer thereof preferably falls between 1/30 and 1/3,more preferably between 1/20 and 1/4. In case where the composite filmhas a three-layered structure of surface layer/inner layer/surfacelayer, the ratio shall be represented by total of the two surfacelayers/inner layer.

[0040] One embodiment of producing the white film for a reflectingstructure for surface light sources of the invention is described below,to which, however, the invention is not limited.

[0041] A composite film-forming device equipped with an extruder A andan extruder B is prepared. To the extruder A, fed is a mixture of 85parts by weight of dry PET chips, 15 parts by weight ofpolymethylpentene, and 1 part by weight of polyethylene glycol having amolecular weight of about 4000. To the extruder B, fed is a mixture of90 parts by weight of PET, 10 parts by weight of calcium carbonateparticles having a mean particle size of about 1 μm, and 0.03 parts byweight of a fluorescent brightener. Needless-to-say, the raw materialcomponents to be fed into the extruders A and B may be previously mixedand pelletized. The extruders A and B are heated at 280 to 300° C., andthe mixtures therein are melted and extruded out in such a manner thatthe inner layer of the melt from the extruder A is sandwiched betweentwo surface layers of the melt from the extruder B to thereby constitutea composite laminate sheet. Then, the thus-extruded sheet is solidifiedon a chill drum having a surface temperature of from 10 to 60° C. Inthis step, preferably, the drum is electrostatically charged so that thesheet can be airtightly held thereon and can be therefore uniformlysolidified thereon. Then, the thus-cooled and solidified sheet is led torolls heated at 70 to 120° C., by which the sheet is stretched about 2to 5 times in the machine direction, and thereafter it is cooled aroundrolls kept at 20 to 40° C. In the continuous line, the film is then ledinto a tenter while its edges are held by clips, in which it ispreheated at 90 to 120° C. and stretched 3 to 6 times in the lateraldirection. Still in the continuous line, the thus-stretched sheet is ledinto a zone heated at 180 to 230° C., in which it is thermoset for about3 to 20 seconds and then cooled to 40° C. or lower. In that manner, thewhite film of the invention is fabricated. To one surface of thethus-fabricated white film, applied is a coating liquid prepared bymixing a UV-absorbing compound, a light stabilizer and a resin in apredetermined ratio, and the thus-coated film is then dried.

[0042] The white film for a reflecting structure for surface lightsources of the invention, thus obtained in the manner as above, has goodinitial brightness and is aged little even in long-term use, and it iseffective for keeping the brightness of liquid crystal display screens.

[0043] [Methods of Measuring and Evaluating Characteristics]

[0044] (1) Mean Void Diameter:

[0045] The cross section of the film is observed at 3,000 to 200,000magnifications, using a transmission electronic microscope, HU-12 Model(by Hitachi). With the voids seen therein being marked, the TEM pictureis analyzed by the use of a high-vision image analyzer, PIAS-IV (byPIAS). Concretely, 100 of the voids are converted into true circles, andthey are averaged to obtain the mean void diameter of the voids.

[0046] (2) Specific Gravity:

[0047] The film is cut into pieces of 50 mm×60 mm each. Using ahigh-precision electronic gravimeter SD-120L (by Mirage Trade), thepiece sample is measured according to the method A (underwaterdisplacement) of JIS K-7112. In measuring it, the temperature is 23° C.and the humidity is 65% RH.

[0048] (3) Glossiness:

[0049] Using a digital varied-angle glossmeter, UGV-5B (by Suga TestInstruments), the film is measured according to JIS Z-8741, with itswhite polyester layer (A) being exposed to light. The incident angle is60°, and the light intercepting angle is 60°.

[0050] (4) Mean Reflectance:

[0051] Using a spectrocolorimeter, SE-2000 Model (by Nippon DenshokuKogyo), the spectral reflectance of the film is measured at intervals of10 nm within a range of from 400 to 700 nm, according to JIS Z-8722. Thedata are averaged, and it indicates the mean reflectance of the film.

[0052] (5) Brightness of Surface Light Source:

[0053] As in the device of FIG. 1, a dot pattern 15 is printed on atransparent acrylic light guide plate 14 of 2 mm thick. On the side ofthe dot pattern printed on the transparent acrylic light guide plate,set is a reflecting sheet 11 made of a film sample; and on the otherside thereof, laminated is a diffuser 13 made of a semi-transparentsheet. A cold cathode-ray tube 16 of 6 W fluorescent lamp is fitted toone end of the transparent light guide plate 14, and it is covered witha reflector 12 as in FIG. 1. The fluorescent lamp is put on, and thebrightness (cd/m²)of this structure is measured on the side of thediffuser 13, using a digital photometer J16 and a brightness probe J6503(by Tectronics). The light interceptor of the brightness probe fitted tothe photometer is applied vertically to the diffuser 13. 9 points in 9uniform sections of the diffuser surface are measured three times each,and the data are averaged.

[0054] (6) Mean Reflectance and Brightness After Durability Test:

[0055] Using a UV aging promotion tester, I-Super UV Tester SUW-W131 (byIwasaki Electric), film samples are forcedly exposed to UV rays underthe condition mentioned below.

[0056] Condition of UV Irradiation:

[0057] Illuminance, 100 mW/cm²; temperature, 60° C.; humidity, 50% RH;time for irradiation, 8 hours.

[0058] After thus exposed to UV rays, the sample films are measured todetermine the mean reflectance and the brightness thereof, according tothe methods (4) and (5).

EXAMPLES

[0059] The invention is described with reference to the followingExamples and Comparative Examples, to which, however, the invention isnot limited.

Example 1

[0060] Raw material mixtures mentioned below were fed into a compositefilm-forming device equipped with an extruder A and an extruder B.

[0061] Extruder A:

[0062] 90 parts by weight of PET chips dried in vacuum at 180° C. for 4hours, 10 parts by weight of polymethylpentene, and 1 part by weight ofpolyethylene glycol having a molecular weight of 4000.

[0063] Extruder B:

[0064] 100 parts by weight of PET chips containing 15% by weight ofbarium sulfate particles having a mean particle size of 1 Am, and driedin vacuum at 180° C. for 4 hours, and 3 parts by weight of PET masterchips containing 1% by weight of a fluorescent brightener, OB-1 (byEastman), and dried in vacuum at 180° C. for 4 hours.

[0065] In the extruders A and B, the mixtures were separately melted andextruded out of them at 290° C. in such a manner that the melt from theextruder A could be an inner layer sandwiched between two outer layersof the melt from the extruder B, and the resulting laminate melt wasextruded out through a T-die into a sheet. The thickness ratio, B/A/B ofthe composite film was 5/90/5. This was cast onto a mirror-face chilldrum having a surface temperature of 20° C. to be a non-stretched sheet.The sheet was pre-heated by rolls heated at 90° C., and then, at 95° C.,it was stretched 3.5 times in the machine direction. One surface of themonoaxially-stretched sheet was corona-discharged in air, and thencoated with a polyurethane emulsion, AP-40 (by Dai-Nippon Ink) to havethereon a coating layer having a dry thickness of 0.3 μm. Next, thesheet was led into a tenter heated at 105° C., with its edges being heldby clips, in which water was removed from the coating layer. In thecontinuous line at 110° C., the sheet was then stretched 3.5 times inthe lateral direction. Still in the continuous line, the sheet wasthermoset at 215° C. for 8 seconds. The white film thus fabricated hadan overall thickness of 188 μm.

[0066] The polyurethane-coated surface of the white film was coated witha coating liquid containing a light stabilizer, U-Double UV6010 (byNippon Shokubai) to form thereon a coating layer having a dry thicknessof 5 μm. Thus coated, this was dried in hot air at 150° C. for 2minutes. The thus-fabricated white film for a reflecting structure forsurface light sources was yellowed little in the durability test, andits mean reflectance reduction and brightness reduction were small, asin Table 1.

Comparative Example 1

[0067] The white film prepared in Example 1 was directly tested for itsapplicability to the white film in the reflecting structure for surfacelight source illustrated. Its initial brightness was good, but itsdurability was extremely poor. Concretely, it was greatly yellowed inthe durability test, and its mean reflectance reduction and brightnessreduction were noticeable.

Examples 2 to 5

[0068] A white film was prepared in the same manner as in Example 1. Thepolyurethane-coated surface of the white film was coated with the samelight stabilizer-containing coating liquid as in Example 1 to formthereon a coating layer having a dry thickness of 1 μm (Example 2), 3 μm(Example 3), 7 μm (Example 4), or 10 μm (Example 5). These samples wereall superior to the sample of Comparative Example 1, though thedurability of the samples of which the thickness of the coating layerwas smaller than that of the sample of Example 1 (Examples 2 and 3)lowered in some degree and the initial brightness of the samples ofwhich the thickness of the coating layer was larger than that of thesample of Example 1 (Examples 4 and 5) lowered in some degree.

Example 6

[0069] A white film was prepared in the same manner as in Example 1. Thepolyurethane-coated surface of the white film was coated with a coatingliquid having the composition mentioned below to form thereon a lightstabilizer-containing coating layer, in the same manner as in Example 1.The thus-fabricated white film for a reflecting structure for surfacelight sources was yellowed little in the durability test, and its meanreflectance reduction and brightness reduction were small, as in Table1.

[0070] (Coating Liquid for Forming Coating Layer) U-Double UV714 (40%solution by Nippon Shokubai) 10.0 g Sumidule N3200 (curing agent, bySumitomo-Bayer Urethane)  0.5 g Ethyl acetate/toluene (1/1 by weight)12.0 g

Examples 7, 8, 9

[0071] A white film was prepared in the same manner as in Example 6. Thepolyurethane-coated surface of the white film was coated with a coatingliquid that had been prepared by mixing 20.0 g of the coating liquid inExample 1 with 0.13 g (solid content, 3% by weight), 0.21 g (solidcontent, 5% by weight) or 0.3 g (solid content, 7% by weight) ofinorganic fine particles of silica powder (Fuji Silicia's SYLOPHOBIC100) with stirring, to thereby form on that surface a lightstabilizer-containing coating layer in the same manner as in Example 6.The thus-fabricated white film for a reflecting structure for surfacelight sources had good mean reflectance and good brightness, and wasyellowed little in the durability test, and its mean reflectancereduction and brightness reduction were small, as in Table 1.

Comparative Example 2

[0072] A single-layer film-forming device equipped with an extruder Aalone was used herein. PET chips containing 10% by weight ofanatase-type titanium oxide particles having a mean particle size of0.15 μm were fully dried in vacuum, fed into the extruder, andmelt-extruded at 290° C. into a sheet. This was cast onto a chill drumat 20° C. to be a non-stretched sheet. The sheet was stretched in themachine direction and in the lateral direction, and then thermoset intoa white film having a thickness of 188 μm, in the same manner as inExample 1. The film was coated with a light stabilizer-containingcoating layer, in the same manner as in Example 1.

[0073] As in Table 1, the coated film had few voids therein, and itsinitial reflectance and brightness were low. TABLE 1 Thickness of LightContent of stabilizer-con- Inorganic Fine Initial Characteristics AfterDurability Test Tested taining Particles in Mean Void Diameter SpecificMean Mean Matters Coating Layer coating layer Inner Layer Surface LayerGravity Glossiness Reflectance Brightness Reflectance Brightness unit μmwt. % μm μm — % % cd/m² % cd/m² Example 1 5 0 28 8 0.83 93 89 561 87 545Co. Ex. 1 — — 28 8 0.83 25 91 573 64 334 Example 2 1 0 28 8 0.83 90 90567 78 497 Example 3 3 0 28 8 0.83 92 90 564 83 525 Example 4 7 0 28 80.83 95 88 554 87 543 Example 5 10  0 28 8 0.83 95 86 538 85 531 Example6 5 0 28 8 0.83 93 86 563 84 548 Example 7 5 3 28 8 0.83 57 87 566 85550 Example 8 5 5 28 8 0.83 41 88 571 85 552 Example 9 5 7 28 8 0.83 3089 574 86 554 Co. Ex 2 5 0 no void — 1.43 71 76 458 74 436

INDUSTRIAL APPLICABILITY

[0074] Exposed to a light source, the white film for a reflectingstructure for surface light sources of the invention is aged little withtime. When built in liquid crystal displays, it ensures good imagequality and brightness of the display screens for a long time.Therefore, the white film of the invention is favorable to reflectingsheets and reflectors in edge light-type and direct back light-typesurface light sources for liquid crystal display screens.

1. A white film for a reflecting structure for surface light sources,which contains voids inside it and has a light stabilizer-containingcoating film formed on at least one surface of it.
 2. The white film fora reflecting structure for surface light sources as claimed in claim 1,wherein the coating layer is formed of a copolymer of an acrylic ormethacrylic resin with a light stabilizer component.
 3. The white filmfor a reflecting structure for surface light sources as claimed in claim1 or 2, wherein the light stabilizer is at least any one of hinderedamines, benzotriazoles, and benzophenones.
 4. The white film for areflecting structure for surface light sources as claimed in any ofclaims 1 to 3, of which the mean reflectance is at least 85%, measuredon the light stabilizer-containing coating layer thereof exposed tolight having a wavelength of from 400 to 700 nm.
 5. The white film for areflecting structure for surface light sources as claimed in any ofclaims 1 to 4, of which the degree of glossiness is at most 60%,measured on the light stabilizer-containing coating layer thereof. 6.The white film for a reflecting structure for surface light sources asclaimed in any of claims 1 to 5, of which the white film is formed of aresin composition consisting essentially of polyester.
 7. The white filmfor a reflecting structure for surface light sources as claimed in anyof claims 1 to 6, in which the voids are formed through melt extrusionof a mixture of a polyester resin, and a resin not miscible with thepolyester resin and/or organic or inorganic fine particles, followed bystretching the sheet in at least one direction.
 8. The white film for areflecting structure for surface light sources as claimed in any ofclaims 1 to 7, of which the white film is a composite film.
 9. The whitefilm for a reflecting structure for surface light sources as claimed inclaim 8, of which the composite layers of the white film containinorganic fine particles and have voids formed from the nuclei of thefine particles therein.
 10. The white film for a reflecting structurefor surface light sources as claimed in claim 8 or 9, of which the whitefilm is a composite film that contains voids in both the surface layerand the inner layer thereof, and in which the mean diameter of the crosssection of the voids is smaller in the surface layer than in the innerlayer.
 11. The white film for a reflecting structure for surface lightsources as claimed in any of claims 1 to 10, in which the coating layeradditionally contains organic and/or inorganic fine particles.
 12. Thewhite film for a reflecting structure for surface light sources asclaimed in any of claims 1 to 11, in which the coating layer and/or thewhite film additionally contains a fluorescent brightener.